THE ALGAE 



131 



divide only perpendicular to the surface. Inside this comes the cortex, 

 composed of elongated cells separated from one another by mucilage, and 

 finally in the middle is the central medulla composed of larger, longitudinally 

 running filaments whose function appears to be primarily that of conducting 

 materials in solution. In the stipe these zones can be clearly seen. 



The cells of the medulla may become greatly modified. This applies 

 particularly to certain cells which cease to divide at an early stage (Fig. 118). 



^j.M* 



^' 



Assimilatory zone 



Outer cortex 



Inner cortex 



Medulla 



They are drawn out by the growth of the tissues, into long straight filaments, 

 considerably attenuated, except at their ends, which retain the original width, 

 thus producing the appearance which has given them the name trumpet 

 hyphae. The end walls are penetrated by numerous protoplasmic con- 

 nections, sometimes sheathed in callus, which later extends to cover both 

 sides of the perforated plate. The structures have been compared to the 

 sieve tubes of higher plants (see p. 467) (Figs. 119 and 120). 



Apart from this the trumpet hyphae possess spiral bands of cellulose 

 thickening on their walls, a feature which again recalls the thickening of a 

 wood vessel (see p. 465). The function of these trumpet hyphae is still 

 uncertain. Manv suggest that they serve for conduction of fluids, while 

 others think of them as storage organs, and others again prefer to regard 

 them mainly as organs of support. It may be pointed out that pitting is not 

 restricted to the trumpet hyphae, as in some species other cells in the inner 

 cortex occur with pitted walls, similar to those in the medulla. These may 

 also facilitate the diffusion of food material. 



A system of anastomosing, intercellular mucilage ducts occurs in the 

 stipe and frond of several species, but only in the fronds of L. saccharina and 



